<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>

        //debugger // 在代码中打了一个断电
        console.log(a);
       
       var a = 1

       console.log(a);

       function a(){
        alert(2)
       console.log(a);
        }
       var a = 3 
       
       console.log(a);

       var a =function(){
        alert(4)
       }

       console.log(a);

       var a 
       
       console.log(a);
    </script>
</head>
<body>
    
</body>
</html>